I don't have data, just after spending time in both cities and going by my own family in Paris I come to that conclusion. One thing that is very noticeable in Paris is that its not just BM and WW that are mixing. My Black female cousin is engaged to a white man. My sister in law is white, my cousin who is mixed black and white is engaged to a half Spanish half Arabic girl.
London is cool don't get me wrong, people mix there and in large numbers but I think Parisians have the edge slightly. Just my own opinion and observations.